The 28-year-old Chelsea striker, left out of the starting line-up for the Group C opener against Italy, returned with a vengeance to spearhead an impressive display by the Spaniards, who ensured Ireland will just be playing for pride against Italy as they cannot qualify from the group stage.
Torres fired them ahead after just four minutes and then, after David Silva had mesmerised the Irish defence to make it 2-0 four minutes after the restart, helped himself to a second to complete a good night's work.
It proved a chastening experience for the Republic, who conceded a fourth late on when substitute Cesc Fabregas collected his second goal of the tournament, as their worst fears were realised.
Spain will head into Monday night's showdown with Croatia with top spot in the group at stake, while Giovanni Trapattoni's men will have only pride to play for when they face Italy, who need to win to progress, in Poznan.
Ireland had gone four games unbeaten before they arrived in Poland, but have tasted comprehensive defeat in both the fixtures they have played in their first trip to major finals in a decade.
The sound of Europe's The Final Countdown thundering from the stadium's PA system 15 minutes before kick-off seemed prophetic at the time, and as Portuguese referee Pedro Proenca brought an end to the opening 45 minutes, the Republic's hopes of extending their stay in Poland and Ukraine were hanging by a thread.
They ran out knowing they needed to avoid defeat to have any chance of reaching the last eight, and that an unlikely win would give them a real chance with Italy and Croatia having drawn earlier.
However, just as they had done against the Croatians in Poznan on Sunday evening, Trapattoni's men found themselves behind barely before they had broken sweat, and the source of the damage was entirely predictable.
Spain boss Vicente Del Bosque had outraged his critics by lining up against the Italians without a recognised striker.
Trapattoni had predicted he would not do the same tonight and would restore Torres to the starting line-up, and the Chelsea frontman needed just four minutes to make his mark.
Silva's fleet-footed burst into the penalty area prompted Richard Dunne to make a desperate and successful sliding challenge, but the ball dropped nicely for Torres to round full-back Stephen Ward and smash a right-foot shot from a tight angle high past goalkeeper Shay Given and into the roof of the net.
It was the last thing Ireland needed, although it came after Simon Cox, preferred to Kevin Doyle in a 4-5-1 formation, had tested Iker Casillas with a second-minute drive from distance.
But Irish forays into enemy territory were to prove a rare occurrence as Spain, not unsurprisingly, dominated the middle of the field to lay siege to Given's goal.
The Aston Villa keeper was peppered from distance as Sergio Busquets, Xabi Alonso and Xavi laid the foundation for Silva, Andres Iniesta and Torres to go about their business.
Given, who had already got down well to claim an effort from Silva, had to beat away a piledriver from Iniesta and in a flurry as the half-time whistle approached, blocked full-back Alvaro Arbeloa's skidding attempt with his legs and tipped another stinging strike from Iniesta over his bar.
By contrast, Robbie Keane, who had worked tirelessly in an attempt to give his side an out-ball, was starved of meaningful possession and found himself chasing a series of lost causes.
Whether Trapattoni felt his decision to use Cox had worked or not, he abandoned the experiment at the break to send on the more physical Jon Walters.
But the game was effectively over within four costly minutes as Spain once again flexed their muscles at the start of the second half.
Given had already repelled another effort from Arbeloa - he and fellow full-back Jordi Alba were effectively playing as wingers - when the keeper's attempt to keep out Iniesta's powerfully-struck shot dropped to the feet of Silva.
The Manchester City midfielder still had plenty to do, but he tip-toed his way through the wreckage of the Irish defence before slotting the ball calmly into the bottom corner.
Given produced a stunning one-handed save to deny Xavi with 55 minutes gone, but would have been picking the ball out of his net once again seven minutes later had Busquets managed to bend his shot only marginally more.
But further damage was inflicted with 20 minutes remaining when Torres once again ran clear of a static defence to fire expertly past Given and finally kill off hopes of a fightback.
Torres departed to rapturous applause from his compatriots with 18 minutes remaining as Fabregas took his place with Spain already looking towards Monday night.
There was an equally warm welcome for James McClean when he was handed a second cap and a first competitive appearance as 76th-minute substitute, but Ireland's woes increased seven minutes from time when Fabregas blasted home a fourth as Trapattoni's men slept at a corner.